Xylem Sap Cytokinin Levels Change with Severe Water Stress in Containerized Citrus Nursery Trees

This experiment was conducted to quantify the effect of water stress on xylem sap cytokinin content in container-grown citrus nursery trees. Trees of Hamlin sweet orange budded on Swingle citrumelo rootstock, were obtained from a commercial citrus nursery. The trees were re-potted in washed quartz sand in 2.65-L citra-pots and were acclimated for 8 weeks. Two sets of 90 trees each were subjected to three water stress treatments (30 trees per treatment): 100% evapotranspiration (ET) (control); 50% ET (mild stress) and 20% ET (severe stress) for 15 days. Stem water potential was measured every other day while photosynthesis measurements were taken on three different days spread across the experimental period. From the first set, five trees were destructively harvested every other day from each of the treatments for xylem sap extraction. To the second set, foliar application of benzyladenine (BA) was given for three consecutive days, starting at day 16 of stress treatments, and the trees were destructively harvested for sap extraction. The xylem sap (800 µl per tree) was extracted using a Scholander-type pressure chamber. Modified Beiliskey’s Solution was added to the sap and the tubes were immediately frozen in liquid nitrogen. The sap samples were analyzed for dihydro-zeatin riboside (DHZR) levels by enzyme linked immunosorbent assay. The stem water potential decreased with the decreasing level of irrigation and with the increasing duration of water stress. The DHZR concentration showed no significant increase with mild water stress, but increased significantly under severe water stress. No correlation was found between foliar application of 100 ppm BA and the DHZR levels over the three day period. However, the trees under water stress showed relatively higher cytokinin levels as compared to the other trees which included the trees under 100% ET as well as those changed from 50% to 100% ET.  The increase in DHZR concentrations may be attributed to a possible stimulation of cytokinin biosynthesis in the root-tips in response to the water stress. DHZR has earlier been reported to increase with water stress in other plant species. However, to determine the implication of altered cytokinin levels on the bud push and scion growth in citrus, the effect of water stress on cytokinin export and delivery rates must be explored.